As is well known during the longwall method of mining, an elongate flexible conveyor extends along the workings adjacent the mineral bearing face. The flexible conveyor is made up of a plurality of conveyor sections, known as pans, bolted together at their ends so that the conveyor can articulate in both vertical and horizontal planes. A mining machine travels up and down the conveyor cutting away a web of mineral with each pass of the machine, and the cut mineral is conveyed to one end of the face by the upper run of the conveyor. Behind the conveyor, and parallel thereto, there is arranged a row of mine roof supports, each connected to the conveyor by a forwardly extending hydraulic ram. Each support comprises a base, a roof engaging canopy and supporting legs or jacks positioned between the base and the canopy.
Once the mining machine has moved past a group of supports, by at least a given distance known as the "snaking distance" (being the linear distance along the length of the conveyor over which the pans are realigned in their advanced location relative to the position the pans were aligned in their previous position, normally over a length of conveyor comprised of nine pans although the greater the advance then the greater the number of pans over which the snaking takes place), the supports push the associated portion of the conveyor forwardly using the advancing rams, and the supports are then sequentially freed from the roof by lowering their legs, and the advancing rams are then used to pull the supports into a new position where they again become adjacent to the conveyor. Thus, as the mining machine moves along the conveyor, the conveyor is gradually moved forward in a snake-like manner, into the newly exposed area created by the passage of the mining machine.
The conveyor is so designed with appropriate spaces between the pans such that during the start of its advance the curvature of the snaking conveyor means that the pan sections abut at their leading edge and are spaced apart at their trailing edge insofar as the bolts connecting the pans will allow, and during a normal advance the angle between the pans will be 6 to 9 degrees maximum, dependent upon the number of pans over which the snake takes place, e.g. 13 or 9. At the halfway position, there is an equi-distant gap between the pans while upon approaching the new aligned position the pans are spaced apart at their leading edge and abut at their trailing edge. During the "snake" sequence, the joints between the pans tend to be stretched such that the pan is in a similar position along the face before and after the conveyor has advanced. If adequate gaps were not present, the conveyor would tend to drift toward one end of the face, taking the roof supports with it. This drift is known as conveyor creep and was extremely common before the introduction of mine roof supports which hold the conveyor against such unwanted movement.
It should be borne in mind that during this snaking sequence the conveyor remains in use, conveying mineral to one end of the face and as such the drive chain and transverse flights scraping elements have to negotiate the snake in the conveyor. The conveyor is further designed to operate on a surface which undulates along the length of the face and the clearance provided between the pans allows this to take place.
For various reasons, all the supports may not move forwardly by exactly the same amount, and equally each portion of the conveyor may not move forwardly by exactly the same amount. Consequently, the conveyor may gradually take up a configuration along its full length which is not absolutely straight, and so the mineral bearing face may eventually become misaligned.
The flexible conveyors referred to typically have endless drive chains which draw the flights along the conveyor, the chains running either centrally or at each end of the flight. The flights and/or chains are restrained by the side walls of the conveyor on both the drive (above the scraping surface) and return runs (below the scraping surface) of the conveyor. Any misalignment of the conveyor pans not only causes excessive wear on the sides of the conveyor but also increases the loadings on the drive motors. Such is the nature of this type of conveyor that an unacceptable degree of misalignment can cause the flights and chain to disengage from the pans which causes a major delay in production and often the need to dismantle the conveyor before normal working can be resumed.
The conveyor also serves as a guide or a track for the mining machine during its passage along the face and any misalignment of the conveyor is often reflected in the straightness of the mineral face itself. This becomes detrimental if the misalignment of the face makes it difficult to maintain a constant distance between the actual face, and the leading part of the roof engaging members of the roof support, thus allowing an unsupported area of roof to develop.
With a view to reducing this problem, it is known to fit one or more of the mine roof supports with a cord transducer which feeds out a length of cord behind the supports as they advance. The length of the cord which is fed out is measured and this information is fed into a computer, enabling the computer to calculate the distance travelled by the supports. This information can be used to control subsequent movement of the supports to ensure that any misalignment of the face is prevented or compensated for. If, for example, a cord transducer is provided on a mine roof support at each end of the face, steps can be taken to control the movement of the end supports so that they move by identical amounts, thus preventing the face from having a skew effect. In addition, further cord transducers can be used at other points in the face to help maintain face straightness and prevent or reduce any bowing effect.
The accuracy of a cord transducer is approximately 0.1 m. The cost of a cord transducer with associated cables is about #3460. A cord transducer is relatively unobtrusive, rugged, intrinsically safe electrically, which is very important in potentially explosive atmospheres, and consumes minimal power. However, the use of cord transducers enables measurement of face alignment only in the plane of face advance.